1. Field of the Invention
The present invention pertains to an improvement made to the guidance, during the approach and landing stages, of aircraft operating in the categories 2 or 3A of the certification system used by recognized civilian organizations such as the DGAC, the FAA or the JAA, this improvement being designed to render the aircraft capable of working in the immediately lower category, namely the 3A or 3B category, at lower cost.
2. Description of the Prior Art
aircraft operating in the categories 2 or 3A require the following conditions during the landing approach stage: a minimum vertical visibility or decision height DH of 50 feet and a minimum horizontal visibility or runway visual range RVR of 200 meters.
These limits of visibility required for operations in the categories 2 or 3A are adopted for aircraft equipped with an automatic pilot controlled by an air data computer or ADC giving the altitude and vertical speed and by an attitude heading reference system or AHRS without inertial platform, associated with a compass giving the longitudinal and transversal attitudes, the values of vertical and horizontal acceleration in all three axes and the magnetic heading.
In spite of very respectable performance characteristics, the 2 or 3A certification levels are not sufficient for certain airlines operating in countries with unfavorable weather conditions. This is because the requirement of meeting the minimum values of visibility laid down by these certification levels obliges them to reroute a considerable proportion of their flights and thus results in a substantial amount of lost business.
Consequently, there is a real need to change the category level of the equipment of an aircraft certified under category 2 or 3A to that of the certification category immediately below, namely 3A or 3B, in order to reduce the minimum levels of visibility and enable landing approaches under conditions of lower visibility.
One way of doing this is to replace the automatic pilot with another one that is certified under a lower category. This operation has the drawback of being costly for it means replacing an attitude heading reference system or AHRS having no inertial platform with an inertial reference system (IRS) type of system.
Another way of providing the requisite additional performance characteristics is to add a head-up display (HUD) visor to the guidance equipment of an aircraft with category 2 or 3A certification in order to form a hybrid system. This method of adding on equipment has the advantage wherein it can take the form of a upgrading kit that can be procured directly by the airline companies and integrated smoothly, without making any major modifications in the architecture of the existing system.
The requirements for category 3A and 3B certification of hybrid systems with head-up display visors are regulated by the certification authorities (especially the JAR AWO and AC 120-28C FAR groups). They entail new constraints, namely the need to have available primary reference information of higher quality and of a nature different from that strictly required under the already available category 2 or category 3A certification, hence information not necessarily available. This would be, for example, information on ground speed vector also called the flight path vector (FPV).
It is therefore necessary to propose a new source for the preparation of these information elements.
This problem is traditionally resolved by the incorporation of an IRS type unit or by an AHRSRS type upgrading, the latter approach involving a recertification of the automatic pilot. Although these approaches are technically satisfactory, their economic value is limited in view of the cost of purchasing and maintaining an inertial reference system.
Besides, it is increasingly being envisaged that aircraft should be equipped with satellite-based GPS (global positioning system) receivers in order to facilitate their localization and navigation. With this in view, an avionics standard (the TSO C 129cl standard) has recently been prepared. This standard sets the minimum characteristics required of a global positioning system GPS receiver on board an aircraft. This standard lays down that the global positioning system GPS receiver, in addition to giving positional coordinates of longitude, latitude and altitude, should also give the east-west and north-south horizontal speeds and the vertical speed of the carrier.